This invention relates generally to semiconductor chip package assembly, and in particular to flip chip package assembly. More specifically, the invention relates to a flip chip package incorporating warpage control structures and methods of their assembly in a semiconductor flip chip package.
In semiconductor device assembly, a semiconductor chip (also referred to as an integrated circuit (IC) chip or “die”) may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g., wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is “flipped” onto its active circuit surface so that the solder balls form electrical connections directly between the chip and conductive pads or traces on a packaging substrate. Semiconductor chips of this type are commonly called “flip chips.”
In a conventional method for packaging a semiconductor flip chip a semiconductor die and a packaging substrate are electrically connected and mechanically bonded in a solder joining operation. The die is aligned with and placed onto a placement site on the packaging substrate such that the die's solder balls are aligned with electrical pads or pre-solder on the substrate. The substrate is typically composed of an organic material or laminate. Heat is applied causing the solder balls to alloy and form electrical connections between the die and the packaging substrate. The package is then cooled to harden the connection.
An underfill is then applied in order to enhance the mechanical bonding of the die and substrate. An underfill material, typically a thermo-set epoxy, is dispensed into the remaining space (or “gap”) between the die and the substrate. The underfill is then cured by heating and then cooling.
Semiconductor packages are typically subject to temperature cycling during normal operation. In order to improve the thermal performance and reliability of the packages, stiffeners and/or heat spreaders are often used. A stiffener may be placed around the die on the substrate where it is bonded with a heat curable organic adhesive. The stiffener (also sometimes referred to as a “picture frame”) is typically a flat piece of high modulus metal about 10 to 40 mils thick, having substantially the same dimensions as the package substrate with a window in its center to clear the die. Typically, the stiffener is composed of nickel-plated copper which has a coefficient of thermal expansion similar to that of typical substrate materials. The stiffener is typically bonded in a separate step following curing of the underfill material. The purpose of the stiffener is to constrain the substrate in order to prevent its warpage or other movement relative to the die which may be caused by thermal cycling during operation of an electronic device in which the package is installed. Such movement may result from the different coefficients of thermal expansion (CTE) of the die and substrate materials, and may produce stress in the die or the package as a whole which can result in electrical and mechanical failures.
A heat spreader (also sometimes referred to as a “lid”), typically composed of a high thermal conductivity material, and having substantially the same dimensions as the package substrate is typically also attached over the stiffener and the die and bonded to the substrate by a thermally conductive organic adhesive. The heat spreader may have a hard or a soft connection with the die via a thermal compound, typically a thermal adhesive (die attach material) or grease, respectively. A conventional heat spreader is also typically a flat piece of the same type of material that is used for the stiffener, for example, nickel-plated copper about 20 to 40 mils thick. A heat spreader may also have a form that allows for direct attachment to the substrate, such as through edges or legs that descend from the flat piece overlying the die to contact the substrate. The heat spreader is usually applied in a separate step following attachment of the die and stiffener, if any. The purpose of the heat spreader is to disperse the heat generated during thermal cycling in order to reduce stress in the package due to different CTEs of the various elements of the package, including the die, substrate and underfill.
A problem with such flip chip package constructions is that during the cool down from the solder joining temperature, the whole package is highly stressed due to the different coefficients of thermal expansion (CTEs) of the substrate and die materials. Shrinkage of the substrate, typically an organic material having a CTE of about 17 ppm, is much more than that of the die, which typically is silicon-based and has a CTE of about 2–3 ppm, e.g., 2.6 ppm. The high stress experienced by these bonded materials during cooling may cause them to warp or crack and cause the package structure to bow. This problem is exacerbated in the case of a relatively large die, for example one 400 mm2 or larger, attached to a relatively small substrate, for example, one 1600 mm2 or smaller. In this case, the bow of the package may exceed the co-planarity specification for packaged flip chips.
FIG. 1 illustrates the problem of semiconductor flip chip package bow due to die/substrate CTE mismatch. A semiconductor flip chip package 100 having a die (flip chip) 102 and substrate 104 electrically connected by solder bumps 106 and mechanically reinforced by underfill 108 is shown. The package 100 also includes a heat spreader lid 112 in contact with the die 102 via a thermal compound 110, usually a grease, and bonded to the substrate 104 with an organic adhesive 114. As shown in FIG. 1, particularly with large die sizes, CTE mismatch between the die and the substrate leads to bowing 120 of the package. For example, in some instances, a substrate in such a package has been known to bow as much as 0.5 mm or more. This amount of bowing 120 exceeds industry specifications (JEDEC (e.g., MS-034), incorporated herein by reference) for the co-planarity required between package substrates and the printed circuit boards (PCBs) 116 to which they are to be attached (e.g., about 0.20 mm for MS-034) via a ball grid array (BGA) 116.
Accordingly, what is needed are flip chip packages and packaging methods that control package bow within acceptable limits for incorporation into electronic devices and to enhance package reliability, particularly for large die sizes.